The invention relates to a circuit arrangement with a relay incorporating a field coil as well as switch contacts, the switch contacts being provided as a switch point between a grid, in particular a mains supply, and an inverter fed from a direct voltage source, in particular from a photovoltaic generator.
Specially devised inverters for photovoltaic plants are known, which have a semiconductor bridge circuit for feeding into a mains supply made from photovoltaic generators. Such a mains supply may be the utility grid or an isolated network. The photovoltaic inverter ensures that the direct current of the source is converted into an alternating current (AC) conforming to the mains.
Such type inverters for feeding into a mains supply are subject to particular demands. They must meet safety regulations, which may vary from one country to another. What is particularly safety-relevant in almost any country is removing the photovoltaic plant, inclusive of the generator and of the inverter, from the grid.
In both sides of AC lines, switches are usually utilized. Due to the magnitude of the current to be switched and to galvanic separation, one uses switch components with switch contacts, in particular with relays.
One problem is that the switch contacts of the relays may get soldered or glued together under certain circumstances, e.g., through failures in the grid. In order to limit this safety risk, it is known to utilize two relays instead of only one so that their switch contacts are connected in series. This series connection is illustrated in FIG. 2.
Whilst dimensions and weight of photovoltaic inverters have remained widely constant over the years, the possible infeed power of the inverters increases considerably in parts. This places a new demand on the component parts used in the inverter. The relays used in the disconnection points should have a space-saving, that is small, configuration and consume little energy.
In conventional inverters one typically utilizes monostable relays for the disconnection points mentioned.
In the unexcited state, such type relays have precisely one, firmly defined switch position. In the normally open type, the contact is for example always open as long as no current flows through the relay coil. Then, closing requires a supply of current. If the switch is mostly closed, it is advantageous to utilize a normally closed type of switch.
On the normally closed type, the contact is closed as long as no current flows through the relay coil. The disadvantage thereof is that it must be supplied with current to be in the opened condition. Accordingly, the normally closed type involves a safety risk. If the current supply fails, the photovoltaic plant cannot be disconnected from the grid.
Moreover, monostable relays suffer from another disadvantage. For one of the two switch conditions, i.e., for opened or closed switch contacts, a permanent current flow is needed. This requires a coil of corresponding size. As a result, the relay is large and expensive.
This is disadvantageous for photovoltaic inverters for feeding into a mains supply. Such type inverters should be low-cost, small and safe, they should consume little energy for their own supply for the efficiency of the plant to be as high as possible.
On the other side, these relays also have advantages. They have simple mechanics, are low-cost and allow for defined basic state of the relay. Moreover, actuation is very simple. If current is interrupted through the field coil, the relay switches.
Bistable relays are known.
In principle, bistable relays meet the demands for little additional space and for low energy consumption. Bistable relays only need a timely limited current pulse in order to change the switch state, in which they then remain. Power is only needed for the switch-over pulse. This is the reason why they need considerably less energy than monostable relays. In contrast to monostable relays, no constant current must flow through the coil in order to keep the relay in an opened or closed position. Since there is no permanent current flow, the field coil does not heat up. A small coil is needed so that the relay itself is small also.
A trigger circuit is known from DE 2747607 C3 in order to also use for monostable switching the advantages of bistable relays such as e.g., low excitation power and lack of unnecessary heating, poor thermoelectric voltage, increase in reliability even of neighbouring component parts and temperature compensation of the excitation voltage. A known arrangement is shown in FIG. 3.
By adding an excitation voltage U to the circuit, the bistable relay R1s is excited and a capacitor C1 is charged at the same time. The relay R1s then switches on. If the capacitor C1 is charged, the current flow stops. By virtue of its bistable property, the relay R1s however remains in its switch position. For switching back by switching the excitation voltage U off, the capacitor C1 discharges via a semiconductor path with a transistor T1 connected in parallel to the relay. The relay R1s is excited in the opposite direction and returns in its position of rest like a monostable relay. In this way, it is possible to operate the relay R1s with minimum trigger energy.